In communications systems, failure to match impedances causes unwanted standing waves resulting from signal reflection from a load. A voltage standing wave ratio (VSWR) is the ratio of the amplitude of a partial standing wave at an antinode (maximum) to the amplitude at an adjacent node (minimum), in an electrical transmission line. FIG. 1 illustrates stages of a communications system, including a variable gain amplifier (Tx), a SAW filter, and power amplifier (PA). The variable gain amplifier adoptively controls power output of a transmitted RF signal. The impedance at the output of the transmitter changes along with the voltage of the variable gain amplifier. FIG. 1 illustrates a situation where VSWR degradation occurs at the output of the transmitter and/or at the output of the power amplifier.
In order to obtain the maximum RF energy transfer between the transmitter (Tx) and the load, it is necessary that the impedance of the load match the output impedance of the transmitter or final RF amplifier. Generally, this load value is specified by the transmitter or amplifier manufacturer as being 50 ohms impedance, resistive, with j=0 where “j” is the notation for the value of reactance, either capacitive or inductive, and is expressed in ohms. This means that that the transmitter “wants” to be connected to a 50 ohms resistance, with no capacitive or inductive reactance. In theory, this is possible; in practice, it is nearly impossible, because all real components have some amount of inductance or capacitance—or both—in addition to their load resistance. If excessive inductive or capacitive reactance is present in the load, it may cause the amplifier to deliver less than its rated power; to draw excessive DC power; and, in some case, possibly overheat the PA amplifier to the point of destruction. The problem can become more complex when variable transmitter gain is necessary for the system communications such as, for example, in Wideband Code Division Multiple Access systems (W-CDMA). The majority of gain control is obtained through voltage gain amplifier parts, which can be built in common emitter configuration with bias current variation. The output network is set to present optimal impedance to the last BJT/CMOS stage in order to deliver maximum power with minimum Icq quiescent current.
A disadvantage of this variable gain amplifier (VGA) structure is that variation due to bias current control for variable gain purposes creates a mismatch at the output of the circuit (transmitter in this case). This variation which creates a high VSWR level, can disturb the rest of the transmitter chain. Particularly, the frequency response of the typical SAW filter used in such system, causes gain slope or phase change and may lead to the communication signal degradation. The last stage PA may face the same issue even without variable gain.
In view of the foregoing, there exists a need for an improved circuit and method for a VSWR compensation circuit at the output port of Variable Gain Amplifier. Without a compensation circuit, the transmit chain may create excessive ripple and may lead to poor communication channel measurements. Furthermore, in high dynamic output power system such as CDMA or W-CDMA, this kind of compensation circuit should be also able to handle high power peak signal without linearity degradation.